Add a parameter "default" to destinations

Created by: christopho

On a map with several destinations, it would be useful to have one of them set as the default one. This can be done with a new boolean property called "default" in destination entities.
It would allow to call hero:transport(map_id) without the destination parameter, making it easier to debug.
And it would also allow to set easily the initial destination when starting a savegame, without having to do it from a script.